Olmesartan Side Effects: What to Expect and When to Call Your Doctor

Olmesartan (Benicar) is generally well-tolerated — in clinical trials, its side effect profile was similar to placebo for most patients. But like all medications, it can cause side effects that range from minor inconveniences to serious medical concerns. Here's what you need to know about olmesartan's safety profile.

Boxed Warning: Fetal Toxicity (Pregnancy)

Olmesartan carries the FDA's strongest warning — a black box — about fetal toxicity. When taken during the second or third trimester of pregnancy, olmesartan can cause serious fetal harm or death, including fetal kidney failure, skeletal defects, limb deformities, and pulmonary hypoplasia. If you become pregnant while taking olmesartan, stop taking it immediately and contact your doctor right away. Women of childbearing age should use effective contraception while taking olmesartan.

Common Side Effects of Olmesartan

The most frequently reported side effects in adults taking olmesartan in clinical trials include:

Dizziness (3%): The most common reported side effect. Often worse when standing up quickly (orthostatic hypotension). Usually improves after a few weeks of treatment.

Headache: Mild headaches are common, especially early in treatment.

Back pain: Reported slightly more frequently with olmesartan than with placebo in trials.

Diarrhea: Mild diarrhea can occur, though severe persistent diarrhea is a red flag (see serious side effects below).

Fatigue: Mild tiredness, especially in the first few weeks as your body adjusts.

Upper respiratory symptoms: Runny nose, sinusitis, and bronchitis have been reported. Unlike ACE inhibitors, olmesartan does NOT cause a dry cough.

Serious Side Effects: Call Your Doctor Immediately

1. Severe or Persistent Diarrhea (Olmesartan-Induced Enteropathy)

This is unique to olmesartan among ARBs. Olmesartan-induced enteropathy (also called sprue-like enteropathy) can develop months to years after starting the medication. Symptoms include severe, chronic watery diarrhea, significant weight loss, nausea, vomiting, and electrolyte abnormalities. It can mimic celiac disease. If these symptoms occur, stop olmesartan and contact your doctor immediately — symptoms typically resolve within weeks of stopping.

2. Hypotension (Dangerously Low Blood Pressure)

Olmesartan lowers blood pressure — which is the goal — but in some patients it can drop too low, especially if you are dehydrated, taking diuretics, or on a low-salt diet. Signs include severe dizziness, lightheadedness, fainting, or feeling very weak. If this happens, lie down and contact your doctor.

3. Kidney Problems

Olmesartan can affect kidney function, particularly in patients with underlying kidney disease, heart failure, or renal artery stenosis. Signs of kidney damage include decreased urination, unusual fatigue or weakness, and swelling in legs or ankles. Your doctor will typically monitor your kidney function and potassium levels with regular blood tests.

4. High Potassium (Hyperkalemia)

Because olmesartan reduces aldosterone secretion, it can cause potassium levels to rise too high. Hyperkalemia can cause nausea, muscle weakness, tingling, chest pain, and irregular heartbeat. It's more common in patients with CKD, those taking potassium-sparing diuretics, or those using potassium salt substitutes.

5. Angioedema (Swelling of Face, Throat, or Lips)

Angioedema is a rare but potentially life-threatening reaction. Signs include sudden swelling of the face, lips, tongue, or throat, difficulty breathing, and hives. This is a medical emergency — call 911 immediately and stop taking olmesartan.

What Olmesartan Does NOT Cause

One of the key advantages of olmesartan (and all ARBs) over ACE inhibitors like lisinopril is that it does not cause a dry, persistent cough. If you were switched from an ACE inhibitor to olmesartan because of cough, you should not experience that side effect with olmesartan.

When to Call Your Doctor

You become pregnant or think you might be pregnant

You develop severe, chronic diarrhea or unexplained weight loss

You feel faint or dizzy when standing up

You notice your urine output decreasing significantly

You experience swelling in your face, lips, or throat (seek emergency care immediately)

For more safety information, see: Olmesartan Drug Interactions: What to Avoid and What to Tell Your Doctor

If you need help finding olmesartan in stock at a pharmacy near you, medfinder can help.